Method of forming a bore

ABSTRACT

A method of forming a supported subterranean well bore in which, in one disclosed embodiment, a first drill bit is mounted on a first string of casing tubulars via a steerable tool, and the drill bit is used to form a first bore. Upon reaching the required depth the casing string is cemented in place to support the formed bore and a second drill bit is mounted on a second casing string and is inserted into the first casing string. The second drill bit is used to drill through the wall of the first casing string and proceed to form a second, deeper bore. Once the second drill bit has reached the required depth, the second casing string is cemented in place to support the second bore.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/785,456filed on Feb. 24, 2004, which claimed priority of GB 0320408.8 filedSep. 1, 2003 and GB 0220201.8 filed Aug. 30, 2002, all of which areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a method of forming a supported bore,and in particular, but not exclusively, to a method of forming asubterranean well bore. The present invention also relates to anapparatus for forming a supported bore.

BACKGRTOUND OF THE INVENTION

In the oil and gas exploration and extraction industry, hydrocarbonscontained within subterranean formations are accessed and flowed tosurface via well bores drilled from surface to intersect the formations.Conventional methods of producing a well bore involve a staged process.Initially, a bore section is drilled using a drill bit mounted on adrill string comprising a combination of drilling pipes and collars,which are used to impart rotation and weight to the drill bit. After acertain depth, however, the walls of the bore are likely to becomeunstable and will eventually collapse if unsupported. Thus, to preventcollapse, the next stage involves pulling the drill bit and the drillstring out of the hole to allow a supporting structure to be set inplace. Often a bore will be drilled to a depth of between 610 to 915 m(2000 to 3000 ft) before support is provided. Thus, pulling the drillstring out of the hole is extremely time consuming, considering thatconventional drilling rigs and platforms can accommodate dismantling andstorage of the drill string in lengths, known as “stands”, ofapproximately 27.5 m (90 ft).

As noted above, once the drill bit and drill string have been removedfrom the hole a supporting structure is set in place. The supportingstructure, generally referred to as the casing or liner, comprises anumber of casing tubulars or liner tubulars which are normally coupledtogether by threaded connectors and extend to the bottom of the hole.Once the casing or liner is run into the bore, a cement slurry is pumpedinto the annulus formed between the wall of the bore and the casing,which cement cures to set and seal the casing in place.

Once the supporting structure is set in place, a slightly smallerdiameter drill bit may then be run in the newly cased hole on the drillstring, which has to be reassembled. Drilling may re-commence once theprevious depth is achieved by the drill bit.

The above procedure is then repeated as necessary until the totalrequired depth of the bore is accomplished, and as will be appreciated,the time taken to pull the drill bit and drill string out of the hole,and to run new casing or liner into the hole will increase after eachdrilling operation due to the increasing depth of the bore.

Furthermore, as the bore is progressively drilled in the mannerdescribed above, various readings must be taken in the bore before thecasing is cemented in place, such as depth, temperature, pressure,formation characteristics and the lice. Acquiring such readings isgenerally termed “logging” or “open hole logging” and is normallycarried out with the drill bit removed from the unsupported hole. Somelogging operations, however, may be achieved with the drill bit locatedin the hole, and even while drilling, by utilizing Logging WhileDrilling (LWD) tools or Measurement While Drilling (MWD) tools, whichcan be elaborate and expensive.

SUMMARY OF THE INVENTION

It among aspects of embodiments of the present invention to obviate, orat least mitigate one of the aforementioned problems.

According to a first aspect of the present invention, there is provideda method of forming a supported bore comprising the steps of:

mounting a first drill bit on a first tubular member;

drilling a first bore to a first depth;

inserting a second drill bit mounted on a second tubular member withinthe first tubular member; and

directing the second drill bit towards a wall portion of the firsttubular member and drilling through said wall portion and drilling asecond bore to a second depth.

According to a second aspect of the present invention, there is provideda drilling assembly comprising a first drill bit mounted on a firsttubular member and a second drill bit mounted on a second tubularmember, wherein at least said first tubular member includes a deflectingmember mounted therein.

Preferably, the first tubular member is cemented or otherwise fixed inthe bore, before or after the second bore is drilled.

Once the second bore is drilled to the required depth, the secondtubular member may then be cemented or otherwise fixed in place withinthe second bore.

Preferably, the second drill bit is directed towards the wall portion ofthe first tubular member by use of a deflecting member mounted withinthe first tubular member.

Preferably also, the first drill bit is located on a steerable toolbefore being mounted on the first tubular member in order to provide thefirst drill bit and tubular member with directional drilling capability.

Thus, a supported bore may be provided by initially drilling a firstbore using the first drill bit mounted on the first tubular member,wherein the first bore may be deviated as required by use of thesteerable tool. Once the first bore has been drilled to the requireddepth, which depth may be dictated by the requirement to support thewall of the bore, a cement slurry or mixture is pumped into the annulusformed between the first tubular member and the bore wall to secure thefirst tubular member in the first bore. The drill bit and steerable toolmay also be cemented in place and therefore become disposed in the bore.

To drill further, the second tubular member and drill bit are run intothe first tubular member until the second drill bit reaches the level ofthe deflecting member, which forces or guides the second drill bittowards the wall of the first tubular member, through which an openingis drilled, allowing the second tubular member and drill bit to exit thefirst tubular member and proceed to drill a second bore. Thus, drillingthrough the wall of the first tubular member avoids the requirement todrill through the first drill bit, and steerable tool if present, todrill to the second depth.

Once the second bore is drilled as required, the second tubular membermay then be cemented in place to support the bore wall. The process maythen be repeated until the required total bore depth is achieved.

By cementing the drill bit supporting tubular members in place to caseor line the bores, the time normally associated with providing a casedbore is greatly reduced as the requirement to pull the drill string anddrill bit out of the hole and then run in a string of casing or a lineris eliminated.

Advantageously, the method according to the first aspect is particularlyadapted for use in producing a supported bore which extends from surfacelevel and intersects a subterranean hydrocarbon bearing formation.

Conveniently, the supported bore may be a deviated bore or amultilateral bore or the like.

The second drill bit may also be located on a steerable tool in order toprovide the second drill bit and tubular member with directionaldrilling capability.

Preferably, the steerable tool is a mechanical device that can beadjusted to effect changes in bore direction. The steerable tool maytake any appropriate form and may be, for example, a directionaldrilling apparatus such as that described in Applicant's UK PatentApplication no. 0212553, the disclosure of which is incorporated hereinby reference.

As noted above, the deflecting member, in use, directs the second drillbit towards the wall portion of the first tubular member, through whicha hole is drilled to allow a second bore to be drilled. The deflectingmember is preferably set at a chosen angle with respect to thelongitudinal axis of the first tubular member. Preferably also, thedeflecting member is fixed relative to the first tubular member. Thedeflecting member may be set at an angle of between 0.5 to 5 degreeswith respect to the longitudinal axis of the first tubular member,resulting, in use, in the second drill bit being deflected from itsinitial path by a corresponding angle.

Preferably, the deflecting member includes a hardened surface to deflectthe second drill bit towards the wall of the first tubular member, andto prevent the member from being destroyed by the drill bit.

Preferably also, the deflecting member defines at least one fluidcommunicating aperture which allows the flow of fluids through and pastthe deflecting member. Such fluids may be drilling fluid or cementslurry or the like.

Conveniently, the deflecting member may be a whipstock or a kick-offplate or the like.

Advantageously, the portion of the wall of the first tubular memberopposing the deflecting member is of a reduced hardness relative to theremaining portion of the first tubular member. This allows the seconddrill bit to more readily drill through the wall of the first tubularmember. The portion of the wall to be drilled may, for example, becomposed of a relatively soft metallic material or a composite materialor the like.

Preferably, the first tubular member is a tubing string or a drillstring comprising at least one, and preferably a plurality of casingtubulars. Alternatively, the tubing or drill string may comprise atleast one liner tubular.

Conveniently, the second tubular member is a tubing string or a drillstring and preferably comprises a plurality of casing tubulars and/orliner tubulars or the like. Alternatively, the second tubular member maycomprise a plurality of drilling tubulars or drilling collars, or acombination thereof

Advantageously, rotation of the drill bit to effect drilling is providedby corresponding rotation of the tubular member upon which it ismounted. Alternatively, rotation of the drill bit may be achieved by useof a downhole drive unit, such as a positive displacement mud motor, forexample.

Advantageously, at least the first tubular member includes a valveassembly for preventing fluids such as cement which are located in theannulus from flowing or being displaced into the tubular member. Thevalve assembly may be a collar having a selectively closable fluidcommunicating throughbore. Preferably, the valve assembly is a floatcollar and is located above the deflecting member.

Preferably, the second tubular member also includes a valve assembly,such as a float collar or the like.

Conveniently, the valve assembly defines a throughbore allowing fluidssuch as cement or drilling fluid which is pumped through the tubularmembers to pass therethrough. Preferably, the through bore of the valveassembly may be selectively closed, by, for example, a plug or dartprovided from surface level. Alternatively, the throughbore may beclosed by a flapper valve or a ball valve or the like.

Preferably, the first tubular member includes means for determining atleast one parameter of the bore.

Preferably also, the second tubular member includes a deflecting memberand means for determining at least one parameter of the bore.

Conveniently, the means for determining at least one parameter of thebore may include a data acquisition apparatus such as a bore loggingapparatus. Advantageously, the data acquisition apparatus may performdata acquisition while the bore is being drilled. Conveniently, alanding joint may be provided on a portion of the corresponding tubularmember in order to provide a means for locating the data acquisitionapparatus within the corresponding tubular member and also for allowingthe acquisition apparatus to be retrieved from within the tubularmember.

In a preferred embodiment of the present invention, the logging toollanding joint is located above the deflecting member and is located in afixed position relative thereto such that the orientation of thedeflecting member, and thus the deflection angle, may be ascertained by,for example, the data acquisition apparatus. Thus, the direction inwhich the second bore will initially be drilled by the second drill bitcan readily be determined by knowing the orientation of the deflectingmember.

Preferably, any data acquisition apparatus located within acorresponding tubular member is retrieved before the tubular member iscemented in place within the bore.

Thus, by providing a data acquisition apparatus of the type describedabove which may acquire data while the bore is being drilled, therequirement to pull the drill bit and corresponding tubular member outof the hole to perform such data acquisition in a separate operation iseliminated.

Conveniently, the first tubular member further includes means fordetermining the orientation of the first drill bit. This may be achievedby use of the data acquisition apparatus, for example. Alternatively,the orientation of the first drill bit may be achieved by use of aMeasurement While Drilling (MWD) apparatus. Alternatively further, thesteerable tool upon which the first drill bit is located may includemeans for directly or indirectly determining the orientation of thefirst drill bit.

Preferably, means are provided for determining the orientation of thesecond drill bit, which means may be included in the second tubularmember and may comprise MWD apparatus or Logging While Drilling (LWTD)apparatus or the like. Alternatively, where the second drill bit islocated on a steerable tool, the orientation of the second drill bit maybe directly or indirectly determined by said steerable tool.

According to a third aspect of the present invention, there is provideda method of forming a supported bore comprising the steps of:

locating a first drill bit on a steerable tool and mounting thesteerable tool and first drill bit on a first tubular member, said firsttubular member including a deflecting member and means for determiningat least one parameter of the bore and the orientation of the drill bit;

drilling a first bore to a first depth;

inserting a second drill bit mounted on a second tubular member withinthe first tubular member; and

drilling through a wall portion of the first tubular member at thelocation of the deflecting member and drilling a second bore to a seconddepth.

According to a fourth aspect of the present invention, there is provideda method of forming a supported bore, said method comprising the stepsof:

locating a first drill bit on a first expandable tubular member havingan upper portion of a first diameter and a lower portion of a second,larger diameter;

drilling a bore with the drill bit mounted on the first expandabletubular member;

pumping cement into an annulus formed between the expandable tubularmember and the wall of the bore; and

expanding the upper portion of the tubular member to a third diameter,greater than the first diameter.

Preferably, the third diameter is substantially equal to the seconddiameter.

According to a fifth aspect of the present invention, there is provideda drilling assembly comprising a first drill bit mounted on a firstexpandable tubular member, wherein said first expandable tubular memberincludes an upper portion of a first diameter and a lower portion of asecond, larger diameter.

Advantageously, the volume of cement pumped into the annulus between thetubular member and the bore wall is selected such that the annulus issubstantially filled with cement when the upper portion of theexpandable tubular member has been expanded.

Preferably, the upper portion of the tubular member is expanded by useof an expansion mandrel forced through the tubular member. Preferably,the mandrel is moved in an upwards direction to expand the tubularmember. Alternatively, the mandrel is moved in a downwards directionthrough the tubular member.

Conveniently, the mandrel is substantially conical or frusto-conical. Itshould be noted, however, that any other shape of mandrel as wouldreadily be selected by a person of skill in the art may be used.

Preferably, the tubular member includes a valve member defining athroughbore through which fluid may pass. Additionally, the mandrel maydefine a fluid transmitting throughbore providing a passage for fluid.Thus, fluid such as drilling fluid or cement may be pumped into thetubular member, pass through the throughbores in the mandrel and thevalve member respectively, and proceed to flow into the annulus betweenthe bore wall and the tubular member.

Conveniently, the valve member is a collar such as a float collar andassists in preventing fluid contained within the annulus from flowing orbeing displaced into the tubular member.

Preferably, the mandrel is initially located within the lower portion ofthe tubular member above the valve member.

Once the required volume of cement is located within the annulus, thethroughbore in the valve member may be closed by a plug or dart providedfrom surface, or alternatively by a flapper valve or a ball valve or thelike. Thus a chamber may be formed between the collar and the mandrel.

Preferably, the method further comprises the step of pressurizing thechamber formed between the mandrel and the valve member, or otherwisecreating a pressure differential across the mandrel, such that themandrel is forced upwards or downwards as required through the tubularmember to effect expansion due to a pressure differential between thefluid above and below the mandrel.

Preferably, by initially pressurizing the chamber, the throughbore inthe mandrel is closed to maximize the above noted pressure differential,The mandrel througbore may be closed by use of a one way pressure valve,for example, such as a flapper valve or the like.

The chamber may be pressurized with a fluid provided from surface, whichfluid may be pumped through the mandrel and into the chamber.Preferably, the mandrel comprises a pumping mechanism to pump fluid intothe chamber Alternatively, a separate pumping unit may be utilized topump fluid into the chamber, which separate pumping unit may be locatedadjacent the mandrel or alternatively at surface level.

Alternatively, or additionally, the mandrel may be forced through thetubular member by pulling from the surface. For example, once theappropriate drill bit has reached the required depth, a support member,such as a string of drill pipe or a reelable support, may be fed downhole and coupled to the mandrel, which support may be used to pull themandrel upwards, to effect, or assist in expanding the tubular member inwhich it is located. Alternatively, or in addition, the support may beutilized to supply pressurized fluid to create a pressure differentialacross the mandrel, or in other embodiments may be utilized to push themandrel downwards to expand the tubular mandrel.

Advantageously, once the tubular member has been expanded as required,the mandrel may be removed therefrom.

Preferably, the method further comprises the step of inserting a seconddrill bit mounted on a second tubular member within the first tubularmember after said first tubular member has been expanded and drillingthrough a wall portion of the first tubular member and subsequentlydrilling a second bore.

Once the second bore is drilled to the required depth, cement may bepumped into an annulus formed between the wall of the second bore andthe second tubular member.

Advantageously, the first expandable tubular member includes adeflecting member, which deflecting member, in use, deflects or guidesthe second drill bit towards a wall portion of the first tubular memberto be drilled. Preferably, the deflection member is located below thevalve member and the mandrel.

Preferably, the deflecting member is set at a chosen angle with respectto the longitudinal axis of the first tubular member. Preferably also,the deflecting member is fixed relative to the first tubular member. Thedrilling member may be set at an angle of between 0.5 to 5 degrees withrespect to the longitudinal axis of the first tubular member, resultingin the second drill bit being deflected by a corresponding angle.

Advantageously, the portion of the wall of the first tubular memberopposing the deflecting member is of a reduced hardness relative to theremaining portion of the first tubular member. This allows the seconddrill bit to more readily drill through the wall of the first tubularmember. The portion of the wall to be drilled may, for example, becomposed of a relatively soft metal material or a composite material orthe like.

Preferably, the deflecting member includes a hardened surface to deflectthe second drill bit towards the wall of the first tubular member, andto prevent the member from being destroyed by the drill bit.

Preferably also, the deflecting member defines at least one fluidcommunicating aperture which allows the flow of fluids through and pastthe deflecting member. Such fluids may be drilling fluid or a cementslurry or the like.

Conveniently, the deflecting member may be a whipstock or a kick-offplate or the like.

In one embodiment of the present invention, the second tubular membermay be expandable and a portion thereof may be expanded to a largerdiameter once the second bore has been drilled to the required depth.The second tubular member may be expanded before cement is pumped intothe annulus between the second bore wall and the second tubular member,or may preferably be expanded after cement is pumped into the annulus.

Preferably, the first tubular member is a tubing string or a drillstring comprising at least one, and preferably a plurality of casingtubulars. Alternatively, the tubing or drill string may comprise atleast one liner tubular.

Conveniently, the second tubular member is a tubing string or a drillstring and preferably comprises a plurality of casing tubulars and/orliner tubulars or the like. Alternatively, the second tubular member maycomprise a plurality of drilling tubulars or drilling collars, or acombination thereof.

Preferably, the first drill bit is located on a steerable tool such thatthe first drill bit and the first tubular member are provided withdirectional drilling capability.

Similarly, the second drill bit may be located on a steerable tool.

In a preferred embodiment, the first tubular member includes means fordetermining at least one parameter of the bore. Conveniently, the meansfor determining at least one parameter of the bore may include a dataacquisition apparatus such as bore logging apparatus. Advantageously,the data acquisition apparatus may perform data acquisition while thebore is being drilled. Conveniently, a landing joint may be provided ona portion of the corresponding tubular member in order to provide ameans for locating the data acquisition apparatus within thecorresponding tubular member and also for allowing the acquisitionapparatus to be retrieved from within the tubular member.

In a preferred embodiment of the present invention, the logging toollanding joint is located above the deflecting member and is located in afixed position relative thereto such that the orientation of thedeflecting member, and thus the deflection angle, may be ascertained by,for example, the data acquisition apparatus. Thus, the direction inwhich the second bore will be drilled by the second drill bit canreadily be determined.

Preferably, any data acquisition apparatus located within acorresponding tubular member is retrieved before the tubular member iscemented in place within the bore.

Conveniently, the first tubular member further includes means fordetermining the orientation of the first drill bit. This may be achievedby use of the data acquisition apparatus, for example. Alternatively,the orientation of the first drill bit may be achieved by use of aMeasurement while Drilling (MWD) apparatus. Alternatively further, thesteerable tool upon which the first drill bit is located may includemeans for directly or indirectly determining the orientation of thefirst drill bit.

Preferably, means are provided for determining the orientation of thesecond drill bit, which means may be included in the second tubularmember and may comprise MWD apparatus or Logging While Drilling (LAD)apparatus or the like. Alternatively, where the second drill bit islocated on a steerable tool, the orientation of the second drill bit maybe directly or indirectly determined by said steerable tool.

Advantageously, rotation of the drill bit to effect drilling is providedby corresponding rotation of the tubular member upon which it ismounted. Alternatively, rotation of the drill bit may be achieved by useof a downhole drive unit, such as a positive displacement mud motor, forexample.

According to a sixth aspect of the present invention, there is provideda method of

forming a supported bore comprising the steps of:

locating a first drill bit on a steerable tool and mounting thesteerable tool and first drill bit on a first expandable tubular member,said first expandable tubular member including a deflecting member andmeans for determining at least one parameter of the bore and theorientation of the drill bit;

drilling a first bore to a first depth;

pumping cement into an annulus formed between the first tubular memberand the wall of the first bore;

expanding a portion of the first expandable tubular member to a largerdiameter;

inserting a second drill bit mounted on a second tubular member withinthe first tubular member;

drilling through a wall portion of the first tubular member at thelocation of the deflecting member and drilling a second bore to a seconddepth; and

cementing the second tubular member in place within the second bore.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described,by way of example only, with reference to the accompanying drawings, inwhich:

FIGS. 1 to 3 are diagrammatic cross-sectional views of a drillingassembly for use in forming a supported bore in accordance with anembodiment of an aspect of the present invention; and

FIGS. 4 to 7 are diagrammatic cross-sectional views of a drillingassembly for use is forming a supported bore in accordance with anembodiment of an alternative aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is first made to FIG. 1 in which there is shown across-sectional view of a drilling assembly 10 for use in forming a wellbore 12 in accordance with one embodiment of an aspect of the presentinvention. The drilling assembly 10 includes a drill bit 14 mounted on atubular member 16 via a steerable tool 18. In the embodiment shown thetubular member 16 is a string of casing tubulars commonly used insubterranean well bores, which casing string extends from surface andprovides rotation and weight to the drill bit 14. The steerable tool 18is a mechanical device that can be adjusted to effect changes in boredirection; that is, the steerable tool 18 provides the drillingapparatus 10 with directional drilling capability.

The drilling apparatus farther includes a deflecting member 20 whichdefines a fluid communicating aperture 22 which allows the flow offluids therethrough. Additionally, a float collar 24 is also provided,located above the deflecting member 20, which float collar also definesa fluid communicating aperture 26. The apertures 22 and 26 allow thepassage of fluids such as a drilling mud or a cement slurry or the like.Furthermore, the drilling assembly 10 further includes a logging toollanding joint 28 located above the float collar 24 and the deflectingmember 20. The function of the deflecting member 20, float collar 24 andlanding joint 28 will become apparent from the subsequent description.

In use, the drilling assembly 10 drills a first bore 12 to a firstdepth, wherein the steerable tool 18 is used to control the direction ofthe bore 12. While drilling, a data acquisition apparatus (not shown) orlogging apparatus is positioned in the tubular member in the region ofthe logging tool landing joint 28, which landing joint 28 provides ameans for locating the data acquisition apparatus within the tubularmember 16, and also for allowing the acquisition apparatus to beretrieved from within the tubular member 16. The data acquisitionapparatus may be used to record data such as bore depth, temperature,pressure and formation characteristics and the like.

Once the first depth is achieved, the data acquisition apparatus isretrieved from the tubular member 16 and cement 34 is pumped through thetubular member, through apertures 22, 26, and into the annulus 30 formedbetween the bore wall 32 and the tubular member 16, as shown in FIG. 2.Once the required volume of cement 34 has been pumped into the annulus30, a dart 36 is provided from surface level which closes the aperture26. Thus, the float collar 24 prevents cement from being displaced backinto the tubular member 16. As shown in FIG. 2, the drill bit 14 andsteerable tool 18 also become cemented in place and therefore remaindisposed in the bore 12. By cementing the tubular member 16 in place asupported bore may be provided without the requirement of pulling thedrill bit and associated drilling string out of the hole and thenrunning in and cementing in place a separate casing or liner string tosupport and seal the bore.

To drill further, a second, slightly smaller diameter drill bit 38mounted on a second tubular member 40 is run into the tubular member 16,as shown in FIG. 3 of the drawings. The second drill bit 38 is rotatedby the second tubular member 40 from surface. The second drill bit 3 8initially drills through the float collar 34 and underlying cement untilreaching the level of the deflecting member 20 which directs the seconddrill bit towards a wall portion 42 of tubular member 16, through whichwall portion 42 a hole is drilled to allow the second drill bit 38 andtubular member 40 to proceed to drill a second bore to a second depth.

The deflecting member 20 is set at a chosen angle with respect to thelongitudinal axis of tubular member 16. It should be noted that theangle of inclination of the deflecting member 20 is exaggerated in thedrawings for illustrative purposes and that the member 20 is likely tobe set at an angle of between 0.5 and 5 degrees. The deflecting member20 includes a hardened surface to deflect the second drill bit 38towards wall portion 42 and to prevent the member 20 from beingdestroyed by the drill bit 38.

The deflecting member 20 is located in a fixed position relative to thelogging tool landing joint 28 (FIG. 1) such that the orientation of thedeflecting member 20, and thus the deflection angle, may be ascertainedby the data acquisition apparatus when in place. Thus, the direction inwhich the second bore will initially be drilled can readily bedetermined by knowing the orientation of the deflecting member 20.

In the embodiment shown, the wall portion 42 to be drilled by the drillbit 38 is of a reduced hardness relative to the remaining portion oftubular member 16 to allow the drill bit 38 to more readily drillthrough the wall of the tubular member 16. The wall portion 42 may becomposed of a relatively soft metallic material or a composite materialor the lice.

Once the second bore has been drilled to a second depth, the seconddrill bit 38 and tubular member 40 are cemented in place in a similarfashion to that described with reference to FIG. 2 in order to supportand seal the second bore. Additionally, the second tubular member 40 mayalso include a deflecting member such that a further bore may be drilledfrom within the second tubular member 40.

As shown in FIG. 3, the second drill bit 38 is mounted on the secondtubular member 40 via a steerable tool 44 to provide the second drillbit 38 with directional drilling capability.

Reference is now made to FIG. 4 in which there is shown a diagrammaticcross-sectional view of a drilling assembly 100 for use in forming awell bore 102 in accordance with an embodiment of another aspect of thepresent invention. The drilling assembly 100 comprises a drill bit 104mounted on an expandable tubular member 106 via a steerable tool 108.The steerable tool 108 is a mechanical device that can be adjusted toeffect changes in bore direction; that is, the steerable tool 108provides the drilling apparatus 100 with directional drillingcapability.

The tubular member 106 is a string of casing tubulars and includes anupper portion 110 of a first diameter and a lower portion 112 of asecond larger diameter.

The drilling apparatus 100 further includes a deflecting member 114located within the lower portion 112, which deflecting member 114defines a fluid communicating aperture 116 which allows the flow offluids therethrough. Additionally, a float collar 118 is also providedwithin the lower portion, above the deflecting member 114, which floatcollar also defines a fluid communicating aperture 120. Furthermore, thedrilling assembly 100 includes a logging tool landing joint 122 locatedin the upper portion 110 of the tubular member 106 above the floatcollar 118 and the deflecting member 114. The function of the deflectingmember 114, float collar 118 and landingjoint 122 are similar to thosedescribed with reference to the embodiment shown in FIGS. 1 to 3, aswill become apparent from the subsequent description.

The drilling assembly 100 also includes an expansion mandrel 124 locatedin a transition region between the upper and lower portions 110, 112. Asshown in FIG. 4, the expansion mandrel 124 defines a fluid communicatingthroughbore 126 allowing for the passage of fluids such as drilling mudor cement or the like.

In use, the drilling assembly 100 is utilized to drill a bore 102 to therequired depth and at the required orientation by employment of thesteerable tool 108. While drilling, a data acquisition apparatus (notshown) is located in the upper portion 110 of the tubular member 106 atthe location of the logging tool landing joint 122 to record well boredata such as temperature and pressure and the like. Additionally, thedata acquisition apparatus may assist in determining the orientation ofthe drill bit 104.

Once the required depth of the bore is achieved, the data acquisitionapparatus is retrieved from the tubular member 106 and a cement slurry130 is pumped into the tubular member 106, through apertures 116, 120and throughbore 126, and into an annulus 132 formed between the borewall 134 and the tubular member 106, as shown in FIG. 5. Once therequired volume of cement 130 has been pumped into the annulus 132, adart 136 is provided from surface level which passes through thethroughbore 126 in the expansion mandrel 124 and closes the aperture 120in the float collar 118, shown in FIG. 6. Thus, the float collar 118prevents cement 130 from being displaced back into the tubular member106. As described above with reference to FIG. 2, the drill bit 104 andsteerable tool 108 also become cemented in place and therefore remaindisposed in the bore 102.

By closing aperture 120 in the float collar 118, a chamber 138 is formedbetween the collar 118 and the mandrel 124. Once the dart 136 is inplace, a fluid is pumped into the chamber 138 through the mandrel 124 topressurize the chamber 138 in order to force the mandrel 124 upwardsthrough the tubular member 106 to expand the upper portion 110, as shownin FIG. 7, before the cement 130 has cured. By initially pressurizingthe chamber 138 a flapper valve 140 closes the throughbore 126 in themandrel to maximize the pressure differential between the fluid aboveand below the mandrel 124.

The chamber 138 is pressurized by fluid provided through a tubularmember 141 which is run in from surface to engage a suitable profile onthe mandrel 124, which fluid is pumped through a further throughbore 142in the mandrel 124. The tubular member 141 may take any appropriateform, and may be a string of drill pipe, a reelable support, such ascoil tubing, or a control line. When a heavier tubular member 141 isutilized, such as a drill pipe string, the member 141 may be utilized topull the mandrel 124 upwardly through the tubular member 106 to assistin expansion of the upper portion 110.

Once the upper portion 110 is fully expanded, the mandrel 124 is removedfrom the tubular member 106 and a structure similar to that shown inFIG. 2 is produced. Further drilling may then be achieved in a similarmanner to that described with reference to FIG. 3, wherein thedeflecting member 114 deflects a further drill bit towards the wall ofthe tabular member 106 to drill through said wall and proceed to drill afurther bore.

It should be obvious to a person of skill in the art that the abovedescribed embodiments are merely exemplary of aspects of the inventionand that various modifications may be made thereto without departingfrom the scope of the present invention. For example, the drill bit maybe directly mounted to the corresponding tubular member and the drillbit may be rotated by downhole drive means such as a positivedisplacement mud motor or the like. Additionally, the second tubularmember 40 shown in FIG. 3 may be a recoverable drill string.

The embodiment shown in FIGS. 1 to 3 may be used in combination withthat shown in FIGS. 4 to 7, wherein a first supported bore is providedusing the drilling assembly 10 (FIG. 1), and the drilling assembly 100(FIG. 4) is run into the first supported bore to form a second supportedbore, wherein the diameter of the second supported bore is maximized byexpansion of tubular member 106.

The tubular member 106 (FIG. 4) may be expanded before cement is pumpedinto the annulus.

In another embodiment, the tubular member 141 may be adapted tocooperate with the aperture 126 and be utilized to deliver the cement130 and the dart 136 from surface. In this case the flapper valve 140and the additional throughbore 142 may be omitted,

1. A method of forming a supported bore comprising the steps of:mounting a first drill bit on a first tubular member; drilling a firstbore to a first depth; inserting a second drill bit mounted on a secondtubular member within the first tubular member; and directing the seconddrill bit towards a wall portion of the first tubular member anddrilling through said wall portion and drilling a second bore to asecond depth.
 2. A drilling assembly comprising a first drill bitmounted on a first tubular member and a second drill bit mounted on asecond tubular member, wherein at least said first tubular memberincludes a deflecting member mounted therein.
 3. A method of forming asupported bore as claimed in claim 1, wherein the first tubular memberis fixed in place in the first bore before the second bore is drilled.4. A method of forming a supported bore as claimed in claim 1, whereinthe first tubular member is fixed in place in the first bore after thesecond bore is drilled.
 5. A method of forming a supported bore asclaimed in claim 3, wherein the first tubular member is cemented inplace in the first bore.
 6. A method of forming a supported bore asclaimed in claim 1, wherein the second tubular member is fixed in placewithin the second bore.
 7. A method of forming a supported bore asclaimed in claim 6, wherein the second tubular member is cemented inplace within the second bore.
 8. A method of forming a supported bore asclaimed in claim 1, wherein the second drill bit is directed towards thewall portion of the first tubular member by use of a deflecting membermounted within the first tubular member.
 9. A method of forming asupported bore as claimed in claim 1, wherein the first drill bit islocated on a steerable tool before being mounted on the first tubularmember in order to provide the first drill bit and tubular member withdirectional drilling capability.
 10. A method of forming a supportedbore as claimed in claim 1, wherein the method is adapted for use inproducing a supported bore which extends from surface level andintersects a subterranean hydrocarbon bearing formation.
 11. A method offorming a supported bore as claimed in claim 1, wherein the supportedbore is a deviated bore.
 12. A method of forming a supported bore asclaimed in claim 1, wherein the supported bore is a multilateral bore.13. A method of forming a supported bore as claimed in claim 1, whereinthe second drill bit is located on a steerable tool in order to providethe second drill bit and tubular member with directional drillingcapability.
 14. A method of forming a supported bore as claimed in claim9, wherein the steerable tool is a mechanical device that can beadjusted to effect changes in bore direction.
 15. A method of forming asupported bore as claimed in claim 8, wherein the deflecting member isset at a chosen angle with respect to the longitudinal axis of the firsttubular member.
 16. A method of forming a supported bore as claimed inclaim 15, wherein the deflecting member is set at an angle of between0.5 and 5 degrees with respect to the longitudinal axis of the firsttubular member.
 17. A method of forming a supported bore as claimed inclaim 8, wherein the deflecting member is fixed relative to the firsttubular member.
 18. A method of forming a supported bore as claimed inclaim 8, wherein the deflecting member includes a hardened surface todeflect the second drill bit towards the wall of the first tubularmember and to prevent the member from being destroyed by the seconddrill bit.
 19. A method of forming a supported bore as claimed in claim8, wherein the deflecting member defines at least one fluidcommunicating aperture which allows the flow of fluids through and pastthe deflecting member.
 20. A method of forming a supported bore asclaimed in claim 8, wherein the deflecting member is a whipstock.
 21. Amethod of forming a supported bore as claimed in claim 8, wherein thedeflecting member is a kick-off plate.
 22. A method of forming asupported bore as claimed in claim 8, wherein the portion of the wall ofthe first tubular member opposing the deflecting member is of a reducedhardness relative to the remaining portion of the first tubular member.23. A method of forming a supported bore as claimed in claim 22, whereinthe portion of the wall of the first tubular member opposing thedeflecting member is composed of a relatively soft metallic material.24. A method of forming a supported bore as claimed in claim 22, whereinthe portion of the wall of the first tubular member opposing thedeflecting member is composed of a composite material.
 25. A method offorming a supported bore as claimed in claim 1, wherein the firsttubular member comprises at least one casing tabular.
 26. A method offorming a supported bore as claimed in claim 1, wherein the firsttubular member comprises a plurality of casing tubulars.
 27. A method offorming a supported bore as claimed in claim 1, wherein the firsttubular member comprises at least one liner tubular.
 28. A method offorming a supported bore as claimed in claim 1, wherein the secondtubular member comprises a plurality of casing tubulars.
 29. A method offorming a supported bore as claimed in claim 1, wherein tile secondtubular member comprises a plurality of liner tubulars.
 30. A method offorming a supported bore as claimed in claim 1, wherein the secondtubular member comprises a plurality of drilling tubulars.
 31. A methodof forming a supported bore as claimed in claim 1, wherein the secondtubular member comprises a plurality of drilling collars.
 32. A methodof forming a supported bore as claimed in claim 1, wherein rotation ofthe drill bit to effect drilling is provided by corresponding rotationof the tubular member upon which it is mounted.
 33. A method of forminga supported bore as claimed in claim 1, wherein rotation of the drillbit is achieved by use of a downhole drive unit.
 34. A method of forminga supported bore as claimed in claim 33, wherein the downhole drive unitis a positive displacement mud motor.
 35. A method of forming asupported bore as claimed in claim 1, wherein the first tubular memberincludes a valve assembly for preventing fluids which are located in anannulus outwith the first tubular member from flowing or being displacedinto the tubular member.
 36. A method of forming a supported bore asclaimed in claim 35, wherein the valve assembly is a collar having aselectively closable fluid communicating throughbore.
 37. A method offorming a supported bore as claimed in claim 35, wherein the valveassembly is a float collar.
 38. A method of forming a supported bore asclaimed in claim 35, wherein the valve assembly is located above thedeflecting member.
 39. A method of forming a supported bore as claimedin claim 1, wherein the second tubular member includes a valve assemblyfor preventing fluids which are located in an annulus outwith the secondtubular member from flowing or being displaced into the tubular member.40. A method of forming a supported bore as claimed in claim 39, whereinthe valve assembly is a collar having a selectively closable fluidcommunicating throughbore.
 41. A method of forming a supported bore asclaimed in claim 35, wherein the valve assembly is a float collar.
 42. Amethod of forming a supported bore as claimed in claim 35, wherein thevalve assembly defines a throughbore allowing fluids such as cement ordrilling fluid which is pumped through the tubular members to passtherethrough.
 43. A method of forming a supported bore as claimed inclaim 42, wherein the throughbore of the valve assembly is selectivelyclosed.
 44. A method of forming a supported bore as claimed in claim 43,wherein the throughbore of the valve assembly is selectively closed by aplug or dart provided from surface level.
 45. A method of forming asupported bore as claimed in claim 43, wherein the throughbore is closedby a flapper valve.
 46. A method of forming a supported bore as claimedin claim 43, wherein the throughbore is closed by a ball valve.
 47. Amethod of forming a supported bore as claimed in claim 1, wherein thefirst tubular member includes means for determining at least oneparameter of the bore.
 48. A method of forming a supported bore asclaimed in claim 1, wherein the second tubular member includes adeflecting member and means for determining at least one parameter ofthe bore.
 49. A method of forming a supported bore as claimed in claim47, wherein the means for determining at least one parameter of the boreinclude a data acquisition apparatus.
 50. A method of forming asupported bore as claimed in claim 49, wherein the data acquisitionapparatus is a bore logging apparatus.
 51. A method of forming asupported bore as claimed in claim 49, wherein the data acquisitionapparatus performs data acquisition while the bore is being drilled. 52.A method of forming a supported bore as claimed in claim 49, wherein alanding joint is provided on a portion of the corresponding tubularmember in order to provide a means for locating the data acquisitionapparatus within the corresponding tubular member and also for allowingthe acquisition apparatus to be retrieved from within the tubularmember.
 53. A method of forming a supported bore as claimed in claim 52,wherein the landing joint landing joint is located above the deflectingmember and is located in a fixed position relative thereto such that theorientation of the deflecting member, and thus the deflection angle, maybe ascertained by the data acquisition apparatus.
 54. A method offorming a supported bore as claimed in claim 49, wherein any dataacquisition apparatus located within a corresponding tubular member isretrieved before the tubular member is fixed in place within the bore.55. A method of forming a supported bore as claimed in claim 1, whereinthe first tubular member further includes means for determining theorientation of the first drill bit.
 56. A method of forming a supportedbore as claimed in claim 55, wherein the orientation of the first drillbit may be determined by use of the data acquisition apparatus.
 57. Amethod of forming a supported bore as claimed in claim 55, wherein theorientation of the first drill bit may be achieved by use of aMeasurement While Drilling (MWD) apparatus.
 58. A method of forming asupported bore as claimed in claim 55, wherein where the drill bit islocated on the steerable tool, the steerable tool includes include meansfor directly or indirectly determining the orientation of the firstdrill bit.
 59. A method of forming a supported bore as claimed in claim1, wherein the second tubular member includes means for determining theorientation of the second drill bit.
 60. A method of forming a supportedbore as claimed in claim 59, wherein the orientation of the second drillbit may be achieved by use of a Measurement While Drilling (MWD)apparatus.
 61. A method of forming a supported bore as claimed in claim59, wherein where the second drill bit is located on a steerable tool,the steerable tool includes include means for directly or indirectlydetermining the orientation of the second drill bit.
 62. A method offorming a supported bore as claimed in claim 59, wherein the orientationof the second drill bit may be achieved by use of a Logging WhileDrilling (LWD) apparatus.
 63. A method of forming a supported borecomprising the steps of: locating a first drill bit on a steerable tooland mounting the steerable tool and first drill bit on a first tubularmember, said first tubular member including a deflecting member andmeans for determining at least one parameter of the bore and theorientation of the drill bit; drilling a first bore to a first depth;inserting a second drill bit mounted on a second tubular member withinthe first tubular member; and drilling through a wall portion of thefirst tubular member at the location of the deflecting member anddrilling a second bore to a second depth.
 64. A method of forming asupported bore, said method comprising the steps of: locating a firstdrill bit on a first expandable tubular member having an upper portionof a first diameter and a lower portion of a second, larger diameter;drilling a bore with the drill bit mounted on the first expandabletubular member; pumping cement into an annulus formed between theexpandable tubular member and the wall of the bore; expanding the upperportion of the tubular member to a third diameter, greater than thefirst diameter; inserting a second drill bit mounted on a second tubularmember within the first tubular member after said first tubular memberhas been expanded; and drilling through a wall portion of the firsttubular member and subsequently drilling a second bore.